1. Field of the Invention
This invention relates generally to a method for forming a crystalline film, and, more particularly, relates to a method for forming a crystalline film of a paramagnetic sodium thallium type intermetallic compound.
2. Prior Art
Crystalline films may generally be formed of those elements and compounds which form bulk crystals. The formation of crystalline films on crystalline substrates of the same material is denoted homoepitaxy. The formation of crystalline films on crystalline substrates of a different material is denoted heteroepitaxy. Under special conditions, crystalline films may be formed on amorphous substrates. The formation of crystalline films is a special discipline apart from bulk crystal formation since circumstances and conditions such as substrate composition, substrate crystallinity and orientation, the respective coefficients of thermal expansion, the adhesion of the film to the substrate or the chemical reaction of the film with the substrate, and the geometry of the film, affect film growth and crystal film characteristics. Such crystalline films have utility for electronics applications since they may be outstanding conductors, semiconductors or insulators and can have precisely tailored electrical properties. In addition, they are known to have particularly useful thermal, magnetic and optical as well as chemical properties.
Numerous techniques are available for forming films from the vapor on a solid substrate. These techniques are used more widely than the techniques of liquid phase epitaxy or solid phase epitaxy. Rheotaxy, the formation of a film on a liquid substrate, is not widely used. The techniques for forming films from the vapor state include chemical vapor deposition, vacuum evaporation and its progeny molecular beam epitaxy, and the various types of sputtering such as ion beam, plasma and magnetron sputtering. Refinements in these techniques have increasingly permitted film thickness, deposition rates and film quality to be controlled. Yet, even with refined film formation techniques there is an inherent progression in the difficulty of film formation from production of an amorphous or microcrystalline material to production of a nonaligned polycrystalline material to production of a crystalline film which is a plurality of aligned crystals or a single crystal. And, the formation of crystalline films which are formed of a plurality of aligned crystals, or better yet, of a single crystal of a particular orientation, has only been possible by the use of carefully prepared substrates of the same or a similar material which have an appropriate crystalline lattice exposed on their surfaces. See I. H. Kahn, "The growth and Structure of Single-crystal Films" in Handbook of Thin Film Technology, pp 10-1 et. seq.
The intermetallic compounds of the sodium thallium type are disclosed in U.S. Pat. No. 4,042,447, and in copending application Ser. No. 789,566, now U.S. Pat. No. 4,115,625, as serving as a substrate for the oriented crystalline overgrowth of silicon. The sodium thallium type intermetallic compounds have a dual diamond crystalline lattice in which the constituent atoms of one element form a diamond lattice which is interpenetrated by a slightly displaced but congruent diamond lattice formed by the constituent atoms of the other element. When crystalline silicon is nucleated and grown on a sodium thallium type substrate the recovered product is suitable as a precursor for photovoltaic devices. The nucleation and growth of silicon can be accomplished on a crystalline specimen which is grown from the bulk. However, for a practicable process it is desirable to nucleate and grow crystalline silicon on large surface areas to produce large area photovolatic devices. The formation of crystalline films of sodium thallium type intermetallic compounds and especially of crystalline films with specific orientation is, therefore, a desirable goal.